/Engine/Source/Runtime/Engine/Classes/Animation/AnimInstance.h
StopAllMontagesByGroupName needs a BlendMode and BlendProfile to function properly if using non-default ones in your montages. If you want default BlendMode/BlendProfiles, you need to update the calling code to do so.
void StopAllMontagesByGroupName ( [FName](API\Runtime\Core\UObject\FName) InGroupName, const [FAlphaBlend](API\Runtime\Engine\FAlphaBlend) & BlendOut )